I have good friends who I love to spend time with, but I never want to work with them.

I had colleagues in other teams I loved to work with, who I never wanted to have in my team (e.g. development vs operations. Really cool for problem solving and interacting on a project basis, but too different in daily habits).

I had colleagues in my team I loved to work with daily, who I never want to start a company with.

Even when you already know a lot about people that doesn't mean they'd be a good fit for a cofounder. But then I've never started a startup, I'm happily employed and don't plan on going back to be my own boss.